Heat is on in Sydney as number of extreme days nudges higher

By Peter Hannam and Nigel Gladstone

The number of hot days in Sydney increased in the past decade across the city, particularly those with extreme heat, an analysis of Bureau of Meteorology data shows.

The tally of days with temperatures of 40 degrees or warmer also got a significant boost last month, with sites such as Penrith Lakes recording four such days in a row.

One way to stay cool in Penrith, one of Sydney's hottest locations.

Photo: Dean Sewell

That series had only happened once before in the Sydney Basin, in February 2009 at Richmond, the bureau said.

Fairfax Media analysed the data at eight sites across Sydney for 2008-17 and compared it with the previous 10 years for a range of threshold temperatures.

Across all eight locations – such as Observatory Hill, Badgerys Creek, Bankstown Airport and Prospect Reservoir – the number of days reaching or exceeding 30, 35, 40, 42 or 45 degrees increased over the decade with just one exception.

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Proportionally, the number of days topping the extreme end of the threshold range increased the most.

For instance, the Richmond RAAF base had about 5 per cent more days reaching 30 degrees or warmer in the more recent decade compared with 1998-2007, or 654 such days compared with 628 days.

The difference, though, reached more than 50 per cent for days of 40 degrees or more at 47 to 30 such days. Similarly, 15 days had equalled or exceeded 42 degrees in the 2008-17 period, compared with 10 in the decade earlier.

Richmond averaged about 5 days of 40 degrees per year over the past decade, a number reached in January alone, as 2018 got off to a scorching start, particularly in Sydney's west.

Beating January 2017 was always going to be tough since that month was the hottest on record for every site in Sydney for mean temperatures, and many set new high marks for maximums too.

Even so, Richmond RAAF, Penrith Lakes and Katoomba were among five locations equalling or setting records for average January maximums last month, running as much as 4 degrees above the long-run norm.

"It used to be less common for Sydney Observatory Hill to record 40 degree days," a bureau spokesman said, noting the CBD site recorded seven such days in the past decade compared with just two days in the prior 10 years.

"But we're also recording temperatures that we have not observed in the entire 159 years of records at Sydney Observatory Hill," the spokesman said.

"These hot days are increasing in their intensity and frequency, with more hot days recorded as well as heatwaves which are also increasing in their intensity, duration and frequency."

The trend towards more extreme heatwaves comes as background temperatures rise as a result of climate change. Australia's mean temperatures have been increasing at the rate of about 0.15 degrees every 10 years in recent decades.

Parramatta was the only exception to the general trend among the eight sites examined.

The number of days there exceeding 30 degrees was marginally lower – 492 to 511 – in the most recent decade compared with 1998-2007, according to Fairfax's analysis. However, higher thresholds, such as above 35 degrees, were breached more often in Parramatta in the past 10 years.

Last month's temperature spikes included 47.3 degrees in Penrith on January 7, a mark only exceeded just once before in the Sydney Basin, at Richmond in 1939.

Perhaps not surprisingly, that day was Sydney's most thirsty in 14 years, with water demand reaching 2.27 billion litres, according to Sydney Water.

For January alone, Greater Sydney's water demand reached an average of 1.872 billion litres, or almost a quarter more than the rolling 10-year average for the month.

For the summer to date, Sydneysiders are using 4 per cent more water than a year earlier, no mean feat as the 2016-17 summer was Sydney's hottest.

Last month's warmth was not contained to Sydney.

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NSW as a whole registered its third-hottest January on record for maximums, with average daytime temperatures running 3.78 degrees above the long-run norm, the bureau said.

Nationally, last month was also Australia's third-warmest January for mean temperatures, running at 1.28 degrees above the 1961-90 average.